Steering-angle sensor giving absolute values

ABSTRACT

The invention relates to a steering angle sensor for motor vehicles. The object of the invention is to be able to provide the absolute value of the steering angle position of the steering wheel within a relatively short angle of rotation of the steering wheel. To this end, the invention proposes featuring two preferably concentric circular tracks (1, 2), wherein the first track (1) is used to determine the relative angular movement of the steering wheel and the second track (2) is used to provide the absolute value of the angle position. Advantageous further developments relate to two different coding options.

BACKGROUND OF THE INVENTION

Motor vehicles are increasingly being equipped with aids which aredesigned to correct and/or prevent incorrect driving behavior on thepart of the person operating the vehicle. For example, one of themeasures currently available prevents locking of braked wheels. Othermeasures consist in providing assistance whenever a vehicle threatens torun off track as a result of a curve being taken too tightly. In orderthat correction moments which keep the vehicle on track can be appliedin conjunction with this so-called yawing moment control system, it isimportant that the control system be informed of the radius within whichthe driver intends to operate the vehicle. This is achieved with thehelp of a steering angle sensor. It should also be noted that the wheelangle is not yet known when the system is switched on, i.e., when poweris supplied to the system by turning the ignition key. Consequently, theactual angle of the wheels cannot be obtained by simply adding orsubtracting relevant incremental angle changes. Instead, continuousabsolute readings must be taken to provide the control system withprecise information on the actual angle of the wheels.

It is known that the aforementioned incremental steering angle sensors,which routinely consist of two sensors, cannot provide an absolutesteering angle on their own. An additional third sensor (zero pointsensor) is needed to provide an absolute steering angle once thecircular scanning track has completed a 360 degree rotational movement.This results from the inclusion of a marking on this absolute trackwhich defines the zero point and which is used to calculate the measuredabsolute reading.

The reference signal obtained in this manner is not confirmed until anadditional 360 degree rotation has been completed. Even in the eventthat the angle of rotation of the steering wheel is very much largerthan the angle of rotation of the applicable wheel, the describedprocedure for obtaining an absolute wheel angle reading is notsufficient for some applications in the vehicle.

The object of the invention is to design a steering angle sensor in sucha way that an absolute steering angle value can be determined withsimple means and after a relatively small rotational movement.

SUMMARY OF THE INVENTION

The object of the invention is achieved by a first track that featuresequidistant teeth, by means of which the size of a relative angularmovement can be determined, and by a second track that is used todetermine the absolute angle position of the steering wheel, wherein thetwo tracks, which are rotatable in relation to a frame, are spatiallyallocated to the steering wheel, the absolute angle position of which isto be determined. Thus, in principle the invention consists in theability to determine the absolute value of the steering angle on thebasis of only two information tracks, wherein the first track is used tomeasure the absolute values which are detectable on the second track.

In an advantageous further development of the invention designed toprovide for greater resolution of the incremental readings taken alongthe first track, the first track (incremental track) is measured by twosensors which scan the evenly spaced flanks of the preferablyrectangular teeth along the first track. The teeth and the correspondinggaps are of identical widths. Assuming that the space between two risingflanks is 360 degrees and, consequently, the space between twosuccessive flanks is 180 degrees, the two sensors should be arranged insuccessive order and 90 degrees apart. Thus, the two sensors facing thefirst track can generate two signals which are offset by 90 degrees; ifthe direction of rotation is reversed, one of these signals undergoes a180 degree phase shift, so that the direction of rotation is alsoclearly detectable.

According to an advantageous further development of the invention, asingle detector which scans the second track and displays the absolutevalue is sufficient for purposes of determining the absolute value.However, for reasons of enhanced safety and greater precision, it may beadvisable to provide a fourth detector which scans the second track.

A special analysis device is recommended for purposes of obtaining anactual display of the absolute steering angle value. In principle, thisinvolves using the teeth on the first track to measure the flank spacingon the second track. Conversely, it is important for the invention thatthere be a clear relationship between the spaces between the successiveflanks on the second track and the absolute value of the steering anglebeing measured. In other words, the width of a tooth on the second trackis a measure for the steering angle which the wheels cover in the spacerequired to measure the tooth.

To ensure that the concrete value of the angle can be easily obtainedthe number of teeth serves as the input value for a table which providesthe value of the angle as the corresponding output value.

The yawing moment control system is particularly important at highspeeds, i.e., speeds at which the steering angle set by the driver islikely to relatively small. To ensure that the absolute value can bequickly provided at such small steering angles, it is recommended as afurther development of the invention that the tooth widths of theindividual teeth on the second track (absolute track) vary to correspondto the applicable measured absolute values, the narrow teeth are placedin the vicinity of the absolute track, which is located in proximity tothe absolute zero value of this track.

Instead of exclusively measuring the width of a tooth on the absolutetrack, with measuring time varying as a factor of tooth width, themeasured arc length of the second track can be used to determine theabsolute value which is always the same. In other words, the incrementaltrack establishes the width of a bit map on the absolute track, with theindividual teeth on the incremental track 1 determining the position ofthe individual bits on the code track 2. Thus, the incremental trackdetermines the positions on the code track where the amplitude readingtaken on that track should be recorded as the bit value of a code.

To allow for easy conversion of the measured code value into a concreteangle value, the coded bit map of the second track is processed by atable transforming it into an absolute steering angle value.

A particularly simple mechanical construction results if the tracksconsist of two concentrically arranged circular protrusions equippedwith teeth. This type of design requires very little space and isrelatively easy to manufacture using plastic casting technology.Allocating 8 bits of an 8-bit code to the second track via two toothperiods of the first track has proven to be particularly advantageous interms of the bit coding. By using two sensors facing the first track,the width of two teeth and the gaps pertaining to these teeth can bedivided into eight time segments to which 8 bits of an 8-bit code areallocated on the second track. This provides for a very straightforwarddesign of the sensor tracks.

To obtain a particularly dense sequence of absolute readings, each ofthe code values, which consists of a predetermined number of bits and isindicated on the second track, differs from the remaining code valuesindicated on the second track. In principle, this measure ensures thatthere are no identical bit sequences (e.g., every 8 bits), regardless ofthe starting point on the second track. Although this process is knownin the art (PWM coding), it has proven to be particularly effective anduseful in connection with the sensor used here.

The number of teeth has been proven effective in terms of practicaldesign. The sensor according to the invention is small in terms of itsspatial dimensions. This allows for inclusion of the volute spring inthe sensor casing, thus largely protecting it from environmentaleffects. Optical detectors, in which the teeth of the rotating tracksshield a glowing diode from a detector or, conversely, in which the gapsbetween the teeth expose the diode, have proven to be particularlyeffective. The resulting changes in illumination are measured by anoptical receiver. However, magnetic detectors (Hall probes) or othersensors may also be used.

The diminutive dimensions of the sensor according to the invention allowfor an advantageous further development of the invention, if the sensoris installed in the casing of a steering column switch of the motorvehicle. This sensor may, for example, encase the steering column of thevehicle and itself be encased by the steering column switch of thevehicle.

An embodiment of the invention is described below on the basis of thefigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts, in outline form, an aerial view of two concentricallyarranged sensor tracks.

FIG. 2 depicts the electric signals emitted by the detectors directedtoward the tracks.

FIG. 3 depicts the layout of the two tracks in one plane in terms of aninitial embodiment.

FIG. 4 depicts the analysis of the impulses generated by scanning thetracks.

FIG. 5 depicts, in a second embodiment, the analysis of the impulsesobtained by scanning the tracks.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts, in outline form, an aerial view of two concentricallyarranged sensor tracks, where the first track (incremental track) ispositioned on the inside and is enclosed by the second track with aspace between the two tracks.

The first track 1 is scanned with two detectors 4 and 5, while a thirddetector scans the second track 2 (absolute value track). The firsttrack has a radius R1, while the second track has a radius R2, so thatthe two tracks remain continually equidistant from one another. In fact,it is possible to scan the incremental track 1 with only one detector.As is evident in FIG. 2, the use of two detectors to scan therectangular teeth 7, each of which is adjacent to uniform gaps, isadvantageous. If a 360 degree angle is assigned to each periodicgap-and-tooth sequence, a 180 degree angle applies to each tooth and toeach gap, as the teeth and gaps are of the same width. If we assign anangle of 4α to the flank space on one tooth or one gap, then the angle αcomprises 45 degrees. The two detectors 4 and 5 are now positioned at adistance of 90 degrees or 2α from the track 1. This ensures that twosignals offset by 90 degrees can be generated at the detectors 4, 5, asdepicted by the two upper signal paths in FIG. 2. The direction ofrotation is clearly detectable, as one of the signals experiences a 180degree phase shift when the direction of rotation is reversed. Formeasurement reasons, the flanks of the code track at the output ofdetector 6 are offset by an angle α in relation to the flanks of the twoincremental tracks (detectors 4 and 5).

It is evident in FIG. 3 that the teeth 9 of the code track (detector 6)vary in length. As each tooth width only occurs once along the entirelength of track 2, the tooth width provides information about theabsolute value of the position of the track in relation to the fixeddetectors. As explained earlier, the position of the code track is ameasure for the angle of the wheels or for the steering angle. As thetooth widths of track 2 are at their smallest when steering angles areclose to 0 degrees, which is most commonly the case and is particularlytrue at high speeds, information on the absolute value of the steeringangle can be obtained relatively quickly.

FIG. 4 shows how the widths of the individual teeth on code track 2 canbe measured. As is evident in FIG. 4, it can be determined at thebeginning of tooth 20 that the end of a tooth 30 has been reached andthat a flank change is occurring. No flank changes are detected whenteeth 21, 22, 23, and 24 are passed subsequently. A flank change on thecode track 2 is not detected until tooth 25 appears on the incrementaltrack 1. This leads to the conclusion that the tooth and/or the adjacentgap on the code track must be larger than the span of four teeth on theincremental track. Theoretically, the measured code tooth may be as wideas six incremental teeth.

Once the length of the applicable tooth on the code track has beenestablished, with allowances made for tolerances, the result is enteredinto a table not shown in the figure, which then yields the absolutevalue of the steering angle. The various widths of the teeth or gaps onthe code track 2 are listed in FIG. 3. In the embodiment based on FIG.3, there are nine teeth on the code track and 180 teeth on theincremental track. The table may consist of an ROM table in theanalyzing microprocessor of the sensor.

Thus, it is evident that the use of the second sensor 5 in conjunctionwith the information track 2 allows for determination of the continuoussector after a maximum rotational movement of only <30 degrees. As aresult, the absolute steering angle values can be provided very quickly.

Thus, the mode of operation is as follows: When the ignition is turnedon, the incremental impulses are counted and the direction of rotationis established. In addition, the light-dark portion of the remaininginformation track is measured using the impulse sequences on track 1;the absolute angle reading is based on the length of the light-darksection. Consequently, a precise absolute steering angle becomesavailable within a very short time after "ignition", while the analyzingmechanism is moved into the so-called ECU, i.e., the attachedmicroprocessor.

In terms of measurement precision, it should be noted that the shortestpossible segments of the code track (track 2) are arranged near thezero-degree position of the steering wheel. This is where the fastestsynchronization and smallest possible absolute error occurs. Thegreatest possible steering wheel movement at which clear information onthe current absolute steering angle cannot be provided is 30 degrees.However, it is possible to obtain an estimate of the absolute steeringangle by applying the appropriate software algorithms.

The code sequence proposed in conjunction with the first embodimentconsists of the following light-dark sequences: 36, 27, 34, 53, 26, 45,23, 43, 15, 17, 22, 41, 16, 13, 31, 33, 62, 51, 21, 42, 14, 44, 24, 12,25, 54, 32, 35. Each pair of numbers only occurs once in this numericalsequence. Thus, a clear allocation of numerical sequence and angleposition is possible during right-to-left movement or when a completelight-dark or dark-light phase is exceeded.

As described below, the use of PWM coding along the perimeter of theinfo track ensures that the minimum rotational movement needed todetermine the sector remains constant. This will be explained on thebasis of a second embodiment.

In the following text, a second embodiment is described on the basis ofFIG. 5. This embodiment is characterized by the fact that the minimumrotational movement needed to determine the applicable absolute angleremains constant. In principle, a certain number of teeth are specifiedon the track 1 and, when these teeth are passed on the first track, thelight-dark changes on the code track 2 are evaluated (absolute track).Thus, the light-dark sequence on the code track corresponds to aspecific code. In other words, whenever a tooth is passed on the firsttrack, the system is able to determine whether the track 2 is light ordark. A digital value of 1 is assigned to light and a digital value of 0to dark. In this manner, we obtain an 8-bit coded number whichrepresents the immediate absolute value of the steering angle. Byselecting suitable digital numbers, it is possible to ensure that adigital number will always result, regardless of which tooth on theincremental track is used to begin the analysis of the correspondingdigital number (8 bits in length) on track 2. Once this digital numberhas been analyzed, the aforementioned ROM memory yields the absolutevalue of the steering angle. Thus, once the ignition has been turned onthe incremental impulses are counted and the direction of rotation isdetermined. Furthermore, the absolute value of the steering angle isdetermined when the light-dark changes on the code track 2 are recordedand decoded via ROM tables in the analyzing microprocessor. In thiscase, a precise absolute steering angle also becomes available shortlyafter ignition. During vehicle operation, the number of increments atthe end of each synchronization determines the steering angle, which isverified constantly with the information from the absolute track. Thus,to a certain extent the sensor is redundant or intrinsically safe. Aclear absolute steering angle is always available once the steeringwheel has complete a 16 degree movement. It is preferable to select aso-called maximum sequence as the code sequence. Its resolution amountsto 8 bits. A complete sequence consists of 180 bits, with each 8-bitsequence only occurring once along the entire perimeter. An additionalincrease in precision can be achieved through the use of a seconddetector on the information track. This applies to both embodiments.Thus, neither of the two embodiments requires a static current supply,and both optical and magnetic detector units can be used.

What is claimed is:
 1. A steering angle sensor for determination of anabsolute value of a steering angle, with a first circular track fordetermining a relative angular movement, the first track including aperiodic tooth cap sequence having adjacent teeth and gaps of equalwidth, the width divided into four equal angular sectors, each sectorhaving a predetermined angle α, a second circular track for determiningan absolute angular position, the two tracks rotatable in relation to aframe and coaxially connected to each other, and two detectors emittingpulse sequences permanently mounted in the frame for measuring therelative angular movement of the first track, the detectors spaced fromone another angularly by an offset angle equal to two times thepredetermined angle α, such that one of the two pulse sequencesexperiences a phase shift when a direction of rotation is reversed.
 2. Asteering angle sensor according to claim 1, wherein the second trackprovides flank spaces increasing in size with an increasing absolutesteering angle.
 3. A steering angle sensor according to claim 1, whereina third detector emitting pulse sequences is provided which scans thesecond track.
 4. A steering angle sensor according to claim 3, wherein afourth detector is provided which scans the second track.
 5. A steeringangle sensor according to claim 1, wherein a sensor analyzing devicecounts the number of flanks of at least one of the pulse sequencesemitted by the detectors of the first track between two measured flanksof the second track.
 6. A steering angle sensor according to claim 3,wherein the number of measured flanks is made available to a table as aninput signal and wherein the table provides an output signalcorresponding to the absolute value of a steering angle.
 7. A steeringangle sensor according to claim 5, wherein the sensor analyzing devicedetermines a simultaneously occurring amplitude value of the secondtrack by means of a predetermined number of flanks of one or both pulsesequences of the first track, and wherein the absolute value of asteering angle is forwarded to a coded bit map of the second track.
 8. Asteering angle sensor according to claim 7, wherein the bit map is madeavailable to a table as an input signal, which issues an output signalthat corresponds to the absolute value of a steering angle.
 9. Asteering angle sensor according to claim 7, wherein each of the codevalues consists of a predetermined number of bits and is arranged on thesecond track and differs from the remaining code values arranged on thesecond track.
 10. A steering angle sensor according to claim 7, wherein8 bits of an 8-bit code are distributed on the second track over twotooth periods of the first track.
 11. A steering angle sensor fordetermination of an absolute value of a steering angle comprising:afirst circular track for determining a relative angular movement, thefirst track including a periodic tooth gap sequence having adjacentteeth and gaps of equal width, the width divided into four equal angularsectors, each sector having a predetermined angle α; a second circulartrack for determining an absolute angular position, the two tracksrotatable in relation to a frame and coaxially connected to each other;and two detectors each emitting a signal mounted in the frame formeasuring the relative angular movement of the first track, thedetectors spaced from one another angularly by an offset angle equal totwo times the predetermined angle α, such that one of the two signalsexperiences a phase shift when a direction of rotation is reversed. 12.The steering angle sensor of claim 11, wherein the second track includesa plurality of teeth with radially extending flank surfaces for eachtooth wherein spaces between adjacent teeth increase in size withrespect to an increasing absolute steering angle.
 13. The steering anglesensor of claim 11, including a third detector for emitting a signal toscan the second track.
 14. The steering angle sensor of claim 13,including a fourth detector for emitting a signal to scan the secondtrack.
 15. The steering angle sensor of claim 13, including a sensoranalyzing device for counting the number of flank surfaces of at leastone of the signals emitted by the detectors of the first track betweentwo measured flank surfaces of the second track.
 16. The steering anglesensor of claim 15, wherein the number of measured flank surfaces iscross-referenced in a look-up table to determine the absolute value of asteering angle.
 17. The steering angle sensor of claim 15, wherein thesensor analyzing device determines a simultaneously occurring amplitudevalue of the second track by means of a predetermined number of flanksurfaces of at least one signal of the first track, and wherein theabsolute value of a steering angle is forwarded to a coded bit map ofthe second track.
 18. The steering angle sensor of claim 17, wherein thecoded bit map is cross-referenced in a look-up table to determine theabsolute value of a steering angle.
 19. The steering angle sensor ofclaim 18, wherein each coded bit map consists of a predetermined numberof bits, is arranged on the second track, and differs from other codedbit maps arranged on the second track.
 20. The steering angle sensor ofclaim 18, wherein 8 bits of an 8-bit code are distributed on the secondtrack over two tooth gap sequences of the first track.